New pathway for pollution may change views of how much mercury is lingering in coastal waters

Researchers from the Woods Hole Oceanographic Institution
(WHOI) have found a new and substantial pathway for mercury pollution flowing
into coastal waters. Marine chemists have detected much more dissolved mercury
entering the ocean through groundwater than from atmospheric and river sources.

Mercury is toxic to animals and humans in large
concentrations, particularly in the form known as methyl mercury, which
accumulates in fish. To date, WHOI researchers examined total mercury, not the
more biologically dangerous form, though that is a logical next step. These
initial findings of mercury moving through the coastal groundwater system are significant
for researchers trying to quantify the impact of mercury in the marine environment.

The lead author of the study is Sharon Bone, a former
undergraduate summer student fellow and research assistant in the laboratory of
WHOI marine chemist Matt Charette. Bone is now a first-year graduate student at
the University of California at Berkeley.

The findings were published online on March 21by the journal Environmental Science and Technology and will appear in a printed
issue later this spring.

Mercury pollution comes mostly from industrial emissions to
the atmosphere, especially from coal burning. After getting into the air,
mercury particles eventually precipitate with rain or snow onto the land or
directly into the oceans. Inland deposits of mercury are also weathered and
carried to the coast in runoff from streams and rivers, where they accumulate
in the sediments that build up along the shoreline.

At the same time, wherever aquifers are connected to the
ocean, fresh groundwater can be discharged and salty sea water can penetrate landward
into groundwaterboth passing through and picking up this mercury embedded in
the sediments. This phenomenon of “submarine groundwater discharge” has been
receiving more attention in recent years because scientists have shown that the
flow of groundwater into the ocean carries a substantial amount of dissolved
nutrients, metals, and trace elements.

“This pathway for delivering nutrients and contaminants into
the ocean has long been overlooked and ignored because it was difficult to
quantify,” said Charette, whose lab has advanced such methods in recent years.
“This study is a first of its kind for quantifying the amount of mercury
flowing out of the system.”

Working in Waquoit Bay in Falmouth,
Mass., Bone and colleagues
started by analyzing cores of coastal sediments, observations from shoreline
wells, and measurements of submarine groundwater flow to determine the amount
of mercury flowing out of the subterranean estuary. Then, while sampling
surface waters, Bone detected concentrations of dissolved mercury in the bay that
were much higher than would be expected from simple atmospheric deposition and
runoff.

Charette and fellow chemist Carl Lamborg of the WHOI
Department of Marine Chemistry and Geochemistry couldn’t believe what they were
finding. “We were surprised by how much mercury we detected in Waquoit Bay,” said Lamborg. “We thought, ‘this
can’t be right,’ and went back to the lab to check the results several times.
We realized that if these numbers are right, then something unusual must be
going on.”

After checking and rechecking their methods and data, the
research team found total mercury concentrations that were an order of
magnitude (at least 10 times) higher than what should be deposited by simple
outfall from the atmosphere, and substantially more mercury than could flow in
from local streams. The source had to be submarine groundwater pushing mercury
out from the sediments.

Once
deposited in water, mercury is often changed by microbes into methyl mercury, a
known nerve toxin that can accumulate in the tissues of marine animals that
consume the microorganisms and smaller fish. Methyl mercury is particularly
dangerous to the developing nervous systems of fetuses, young children, and
animals. In recent years, researchers have cautioned pregnant women to limit
the consumption of certain types of fish due to concerns about mercury
contamination.

Charette
and colleagues hope to return to Waquoit
Bay and other sites to
examine which forms of mercuryincluding toxic methyl mercuryare present, in
what concentrations, and how they are moving through the environment. They
would also like to see studies of the effect of the mercury on the biological
systems in the area.

Funding for this research was provided by
the Chemical Oceanography division of the National Science Foundation, the WHOI
Postdoctoral Scholarship program, and the WHOI Summer Student Fellowship
program. WHOI would also like to thank the Waquoit Bay National Estuarine Research Reserve for its support as the host site for this research.

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The Woods Hole Oceanographic Institution is a private, independent
organization in Falmouth, Mass., dedicated to marine research,
engineering, and higher education. Established in 1930 on a
recommendation from the National Academy of Sciences, its primary
mission is to understand the oceans and their interaction with the
Earth as a whole, and to communicate a basic understanding of the
ocean's role in the changing global environment.

Originally published: March 20, 2007

WHOI is the world's leading non-profit oceanographic research organization. Our mission is to explore and understand the ocean and to educate scientists, students, decision-makers, and the public.